Light scanning units that scan laser beams on a photosensitive drum to form electrostatic latent images are generally employed in image forming apparatuses, such as laser printers, copying machines, facsimile machines, or the like that reproduce images on printing media.
A conventional light scanning unit may include an optical unit having a light source emitting laser beams, a polygon mirror, and a synchronization detection optical system to obtain horizontal synchronization signals of light that is scanned according to the rotational direction of the polygon mirror. When the rotational direction of the polygon mirror varies (i.e. clockwise or counter-clockwise), an f-θ lens needs to be enlarged or the size of the polygon mirror needs to be increased, in order to allow the synchronization detection optical system to operate in the various directions. Also, in the case of two light scanning units operating around one polygon mirror and employing a cross-scanning method, synchronization detection optical systems are required for each scanning direction. This results in the same problem regarding the necessity of a larger f-θ lens or larger polygon mirror. Therefore, it is desirable to design a light scanning unit in which the rotational direction of the polygon mirror varies and that does not require large f-θ lenses or polygon mirrors.